Define a "GUI" - web, native, or non-native

Define "complex" - is that 38 elements, 130, 510, or 927? Do modals count?

Define a "line" - JS can be minified, Elm is terse. Do you count libraries?

The problem is you're speaking in such vague, ambiguous generalities that you're also falling into the trap of focusing on having "impressive" lengths rather than the nuances of making things in the real world.

Back in the days of Delphi, that was trivial. The description of the GUI itself was kept separate (I believe in binary files?) and you basically only used code for interaction/changes. A lot of quite complex apps were written in it before Microsoft and their own stupid decisions basically killed them off.